approxFactorial.js

function realFactorial(n) {
  let result = n;

  for (let i = 1; i < n; i++) {
    result *= i;
  }

  return result;
}


function approxFactorial(n) {
  return Math.sqrt(2 * Math.PI * n) * Math.pow(n / Math.E, n);
}

let n = 20;
let a = realFactorial(n)
let b = approxFactorial(n);
// I was calculating this backwards. The error rate actually converges *toward* zero. 😂
// Thanks to @etzpcm
let percentOff = (a - b) / a * 100;

console.log(`${a} ~ ${b} (off by ${percentOff.toFixed(2)}%)`);